Method for producing a shielding gasket

ABSTRACT

The invention relates to a method for producing an electromagnetic shielding gasket by using a shielding compound that comprises a silicone plastic compound, electrically conducting parts and parts that expand under the influence of heat. The shielding compound is dispensed onto a housing and/or a circuit board and/or parts of the housing and is heat-treated after and/or during application, thereby providing the dispensed compound with the desired predetermined expansion and/or shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Not Applicable

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing an electromagneticshielding gasket.

2. Description of the Related Art

Electromagnetic shielding gaskets as well as methods for theirproduction have long been known. In this connection, reference is madeto the different applications of the present applicant as well as to thepublications cited in the examination process relating to theseapplications.

Further, reference is made to the European patent application EP 0 779629 A2. This application describes an electrically conducting compositematerial that has a polytetrafluoroethylene matrix as well aselectrically conducting components and additional hollow polymercomponents that are expandable under the influence of heat. Also statedthere is that the electrically conducting composite is to contain inaddition an elastomeric material, which can consist of a siliconematerial. Although known from the above-named publication is theapplication of the material described and claimed there for theproduction of an electromagnetic shielding gasket, nevertheless it isvery difficult, using the object described in EP 0779 629 A2, to producea shielding gasket, particularly when the external tolerances areextremely small, for example in the range of a few millimeters ofdiameter.

The invention is based on the task of avoiding the disadvantages of theshielding gasket and of its production as known from EP 0 770 629 A2.

BRIEF SUMMARY OF THE INVENTION

The present embodiment is a method for producing an electromagneticshielding gasket, comprising establishing a predetermined form for theelectromagnetic shielding gasket; dispensing a shielding substance as atleast one bead, wherein the shielding substance comprises a siliconeplastic mass, electrically-conducting components and components that areexpandable under the influence of heat; and treating the dispensedshielding substance with heat, thereby deforming the expandablecomponents in the dispensed shielding substance, until the dispensedshielding substance substantially resembles the predetermined form.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 a illustrates a shielding substance in the presence of a guidingwall, before expansion.

FIG. 1 b illustrates a shielding substance in the presence of a guidingwall, after expansion.

FIG. 2 a illustrates a gasket in the presence of multiple guiding walls,before expansion.

FIG. 2 b illustrates a gasket in the presence of multiple guiding walls,before expansion.

FIG. 2 c illustrates a gasket in the presence of multiple guiding walls,after expansion.

FIG. 3 illustrates a gasket forming a varying profile in response to alocally varying heat source.

FIG. 4 illustrates a stamped gasket formed by a prior art method.

FIG. 5 illustrates a dispensed gasket in accordance with the presentembodiment.

FIG. 6 a illustrates a shielding substance dispensed as multiple,relatively small beads.

FIG. 6 b illustrates a shielding substance dispensed as multiple,relatively large beads.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the task is accomplished with a methodaccording to claim 1 and with an electrical apparatus having acorresponding shielding gasket according to claim 6. Advantageousfurther developments are described in the dependent claims.

The essential difference between the present invention and the prior artaccording to EP 0 779 629 A2 consists in the fact that, in theinvention, no polytetrafluoroethylene matrix is necessary for theconstruction of the shielding gasket. Rather, the electrical shieldinggasket is produced through the dispensing of a shielding substance and asubsequent heat treatment of the dispensed compound, the heat treatmenttaking place until the dispensed compound obtains its predeterminedexpansion or form. The shielding substance contains electricallyconducting components, for example silver particle, coal, metallicpowder, etc., as well as expandable components, preferably expandablepolymeric hollow spheres as well as silicone plastic.

The production of a dispensed shielding gasket is considerably moreconvenient and simple than is the production of a gasket as known fromEP 0 779 629 A2.

In the method according to the invention, dispensing devices can be usedfor the dispensed application of the shielding substance and, after thedispensing, the heat treatment can take place either directly during thedispensing process or subsequently thereto in a heating chamber.

The micro hollow spheres can, for example, be such as are available fromthe firm Nobel Industries of Sundsvall, Sweden under the trade names “.. . ”. Such micro hollow spheres can be obtained in a great variabilityof size and form with an expansion temperature lying primarily in therange between 70 and 130° C. A typical . . . micro hollow sphere has aninitial-diameter dimension (i.e. not yet expanded) of 9 to 17 μm and, onaverage, an expanded diameter of 40 to 60 μm. Accordingly, thecubic-meter weight of the micro hollow spheres in the unexpanded stateis approximately 1250 to 1300 kg/m³, and in the expanded stateapproximately 20 to 30 kg/m³.

In particular when the heat treatment takes place during the dispensingprocess of the shielding substance or shortly thereafter, it is possibleto produce a large number of shielding gaskets within the shortestperiod of time in a simple manner, in which the dispensing robot, bymeans of which the shielding substance is applied to a housing, printedcircuit board, or housing part, can easily adjust the dispensedapplication to the demands on the housing in each case. Also, suchdispensing devices can be provided in a simple manner with aheat-generating apparatus, for example an infrared-radiation unit or alaser or another apparatus for heat generation, so that the expandablemicro hollow spheres can expand.

The duration of the heat treatment is dependent on how far the dispensedshielding substance is to expand and what shape it must assume in theprocess.

After the heat treatment, the diameter of the shielding substance, whichis applied, for example, as a bead or beads, is distinctly greater thanprior to the heat treatment. The diameter expansion or widening of thediameter can be adjusted through the temperature and/or duration of theheat treatment such that the desired degree of expansion is achieved.

In particular when the dispensed application is not sufficient whenusing a conventional dispensed bead, for example, because the dispensedbead cannot completely bridge the gap between two housing parts, the useof the solution according to the invention is very expedient andcost-effective. The gasket according to the invention is especiallysuitable for waterproof sealing, whether for spray or even pressurizedwater. Also, it is possible to construct so-called guide walls, i.e.arranging crosspieces and flanges such that when an expansion occurs,the direction and/or form is roughly predetermined. Examples of this arerepresented in FIGS. 1, 2, 3, and 6.

The proportion of silicone (rubber) is the shielding substance can liein the range between 20 and 70%, the proportion of the electricallyconducting components in the range between 20 and 80% by volume(according to the fill material) of the output material, and theproportion of the micro hollow spheres in the range betweenapproximately 1 and 25% by volume of the output material.

After the heat treatment the hardness grade (according to Shore A) ofthe shielding gasket according to the invention amounts to approximately15 to 85; the proper hardness grade can be determined through selectionof an appropriate plastic or duration of the heat treatment.

The absolute diameter of the shielding gasket can lie in the range ofless than 1 mm to 30 mm.

The heat treatment for expansion of the entire shielding substance onthe basis of the simultaneous expansion of the micro hollow spheres canalso take place after the dispensing of the shielding substance andafter the assembly of the housing, for example in a heat oven. Since thetemperatures of the heat treatment are still relatively low and normallycause no destruction of the housing or housing part, such a treatment ofthe already-assembled housing results in the fact that through theexpansion of the shielding substance, the latter is preciselydistributed in the entire housing or in the gap between the adjacenthousing parts, thus closing and sealing the housing such thatelectromagnetic radiation can no longer penetrate into the housing orescape from it. Moreover, at the same time an outstanding mechanicalseal against moisture, liquid, or other objects such as dust isachieved.

After the expansion process, the shielding gasket is, as before, stillelastic and even multiple openings and closings of the housing and thereassembly of housing parts is readily possible without causing damageto the corresponding shielding substance or significantly affecting thefunctionality of the shielding substance.

The heat treatment described in the present application can be carriedout by means of a heated needle or nozzles or, if the gasket is alreadysituated on a part, a simple heat aftertreatment can also be carriedout. It is also possible, using the method according to the invention,to produce an electrically conducting foam, which can be dispensed ontoa foil, for example.

The essential difference between the method according to the inventionand the proposal according to EP 0 779 629—FIG. 4—consists also in thefact that the known production method according to EP 0 779 629 requiressix productions steps and is therefore very expensive. Essentially, thesix production steps are drying (after the mixing), freezing, grindingup, extruding, pressing, and then applying to a foil, e.g. a telephonepart. In the method according to the invention, in contrast, only theproduction steps mixing, applying, and expanding are required.

In comparison to PTFE (polytetrafluoroethylene), as disclosed in EP 0779 629, silicone has a considerably better compression set value. Ingeneral, dispensed gaskets (see FIG. 5) also require lower pressingforces for deformation.

While according to the cited EP 0 779 629 silicone polymers aredescribed as an admixture to the PTFE (polytetrafluoroethylene) mass,the electrically-conducting PTFE compounds also containing an elastomer,and in the production the individual components are mixed, dried (at105° C. for 24 hours) and/or deep-frozen (−10° C. for 6 hours), sieved,diluted, stored at room temperature, extruded, dried, and expanded, themethod according to the invention requires merely a mixing and thefilling into cartridges of the gasket components according to theinvention, and subsequently an application of the gasket, for example bymeans of dispensing using needles and nozzles, can take place. Theexpansion of the shielding substance through heat can occur eitherthrough a heating of the needle (nozzle) itself or the heat is appliedafter the application of the gasket.

Preferably, in the method according to the invention, after thedispensing a gasket with a semicircular cross section is present, whichreduces the force necessary to deform the gasket later.

It is also important to state that in the method according to theinvention the final shape of the gasket is, in all essentials,determined by the type and/or amount of the applied heat. In contrast tothis, known from WO 98/08365 is a method in which the final gasket shape(in particular with respect to its cross section) is determined throughthe needle diameter (nozzle diameter), the application speed, or thoughthe composition of the compound (viscosity, thixotropy, etc.) and/orthrough the application apparatus. Moreover, although the treatment ofgaskets with heat after their application was previously generallyknown, this heat treatment nevertheless served only the purpose ofcross-linking, drying, or hardening the silicone polymers, and hadnothing to do with the concrete formation of the cross-sectional shapeof the gaskets. The determination of the expansion and/or the shape ofthe shielding gasket in dependence on the concrete application of heatis the particular knowledge of the present invention and facilitates theproduction of an electromagnetic shielding gasket in particularapplication forms and, in comparison to the known solutions, makes thisproduction more cost-efficient.

1. A method for producing an electromagnetic shielding gasket,comprising: establishing a predetermined form for the electromagneticshielding gasket; dispensing a shielding substance as at least one bead,wherein the shielding substance comprises a silicone plastic mass,electrically-conducting components and components that are expandableunder the influence of heat; and treating the dispensed shieldingsubstance with heat, thereby deforming the expandable components in thedispensed shielding substance, until the dispensed shielding substancesubstantially resembles the predetermined form.
 2. The method of claim1, wherein the expandable components have a diameter in the range ofapproximately 2 to 50 μm in the non-expanded state, and have a diameterin the range of approximately 30 to 200 μm in the expanded state.
 3. Themethod of claim 1, wherein the expandable components are micro hollowpolymeric spheres, which are electrically nonconductive and areenlargeable in size through heat treatment by a factor in the range ofapproximately 5 to 70 in volume.
 4. The method of claim 1, whereinduring the heat treatment the temperature of the dispensed shieldingsubstance is in the range of approximately 50° C. to 140° C.
 5. Themethod of claim 1, wherein after the dispensing and after the heattreatment the dispensed shielding substance is essentially circular incross section.
 6. The method of claim 1, further comprising providingguide walls for the dispensed shielding substance during the heattreatment, so that the dispensed shielding substance expands around theguide walls and assumes a shape roughly predetermined by the guidewalls.
 7. The method of claim 1, wherein different portions of thedispensed shielding substance undergo a different heat treatment, sothat the dispensed shielding substance may achieve local expansions thatallow for more complex final shapes and may lower the possible physicaldeformation forces.